typeof usage for undefined variables

Question

what is the best usage for the "typeof" JavaScript function?

if (typeof (myvar) == 'undefined') { 
//or
if (typeof (myvar) == undefined) { 
//or
if (typeof myvar == 'undefined') { 
//or
if (typeof myvar == undefined) {

Thanks

why are you doing this? you should just do myvar === undefined. — Daniel A. White
– Daniel A. White, Commented Oct 7, 2011 at 17:02
possible duplicate of How can I determine if a JavaScript variable is defined in a page? — Vivin Paliath
– Vivin Paliath, Commented Oct 7, 2011 at 17:03
@DanielA.White If myvar hasn't been declared, your code would throw ReferenceError. — duri
– duri, Commented Oct 7, 2011 at 17:03

Quentin · Accepted Answer · 2011-10-07 17:02:41Z

14

typeof is an operator, not a function, and returns a string; so do not use parentheses and do compare it to a string.

When you compare things, avoid type coercion unless you need it (i.e. use === not ==).

if (typeof myvar === 'undefined') {

answered Oct 7, 2011 at 17:02

Quentin

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Michael Berkowski · Accepted Answer · 2011-10-07 17:02:35Z

4

Use strict comparison (===), and quote "undefined":

if (typeof myvar === "undefined") {}

answered Oct 7, 2011 at 17:02

Michael Berkowski

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